Two existing drive train configurations for hybrid electric vehicles are known as a “parallel” configuration and a “series” configuration.
In the parallel configuration, an internal combustion engine is mechanically coupled to drive the wheels and also to drive an electrical machine. The electrical machine is connected, by way of power electronics to electrical energy storage means such as a battery or an arrangement of super-capacitors or ultra-capacitors. The electrical machine can be selectively operated as a generator or a motor. When operated as a generator, the electrical machine is driven by the engine to charge the battery. When operated as a motor to drive the wheels, either together with or instead of the engine, the electrical machine discharges the battery.
In the series configuration, the engine is not mechanically coupled to the wheels which are instead always driven by an electrical machine operating as a motor. An example of an existing series configuration 10 is shown in FIG. 1 of the drawings. As can be seen from FIG. 1, the series drive train 10 includes an internal combustion engine 20 mechanically coupled to drive a first electrical machine 30, which operates as a generator. The output of the first electrical machine 30 is connected via first power electronics 40 to electrical energy storage means 50. The electrical energy storage means 50 are also connected via second power electronics 60 to a second electrical machine 70. The second electrical machine 70 operates as a motor and so is mechanically coupled to wheels 80. Whilst a batteries and/or super or ultra-capacitors may be used as the storage means in this arrangement, the rate at which energy can be put into capacitors and removed therefrom makes these more attractive for use as the storage means in at least some applications.
In operation, the various components are operated under the control of a vehicle control unit (VCU) 90. The engine 20 is operated to drive the generator 30 to charge the storage means 50. However, where the storage means 50 include super or ultra-capacitors, it will be appreciated that the state of charge of these capacitors is proportional to the square of the voltage (E=½CV2). The voltage across the capacitors therefore changes considerable with their state of charge. In order to provide for this change in voltage, the first power electronics 40 are provided to control the output voltage of the generator 30 such that it can be used to charge the storage means 50. The first power electronics usually comprise a DC-to-DC converter, which can account for a significant percentage of the cost, and a significant part of the weight, of the drive train components. This is a drawback with the use of super or ultra-capacitors and, to a lesser extent, with electrochemical storage means such as batteries.
The wheels 80 are driven by the motor 70 operating to discharge the storage means 50. Again, the second power electronics 60 are provided to convert the output of the storage means 50 to the input required to operate the motor 70. As the voltage of the storage means 50 is usually higher than that needed to operate the motor 70, simple voltage reduction is all that is necessary and so the second power electronics are typically less expensive and lighter than the first power electronics.
While hybrid electric vehicles have received much attention as being a possible way of reducing the environmental impact of automotive vehicles on the environment—for example through increased fuel efficiency—such vehicles currently account for a very small proportion of total automotive vehicle sales. One of the main reasons for this is the high cost of current hybrid vehicles in comparison with conventional automotive vehicles.
Hybrid drive trains have also been used in motor sport applications. While the high cost of such drive trains is less of an impediment in motor sport, high weight is seen very much as a drawback.
It is therefore desirable to provide an improved drive train for a hybrid electric vehicle.